The present invention relates generally to techniques for removing noise in power amplifiers comprising class-D amplifiers and more particularly to a common mode choke coil apparatus for use in a power amplifier.
In the field of audio amplifiers, there have heretofore been known class-D amplifiers using a class-D amplification method. The class-D amplifier is an amplifier for amplifying electric power on the basis of the class-D operation scheme or mode, which converts an input analog sound signal into a pulse width modulated signal (PWM signal) and performs ON-OFF control on an output-stage switching element in accordance with the pulse width modulated signal. Low-pass filter is provided at a stage following the output-stage switching element, and this low-pass filter removes a carrier frequency included in an output switching signal to demodulate the output switching signal to the analog sound signal and then outputs the demodulated analog sound signal to a load, such as a speaker.
As well known in the art, among various schemes or modes for connecting a speaker to an audio amplifier are a single-end connection (half-bridge connection) mode and a BTL connection (full-bridge connection) mode; “BTL” is an acronym for Bridge-Tied Load or Balanced Transformer Less).
In the case of the single-end connection, one speaker is connected to a class-D amplifier of one channel, and one of terminals of the speaker is connected to the ground. Where a power amplifier for handling stereo signals of two channels is to be constructed using such class-D amplifiers designed for single-end connection (hereinafter sometimes referred to as “single-end connecting class-D amplifiers”), two class-D amplifiers are provided in one-to-one corresponding relation to the two channels (see, for example, FIG. 1 of Japanese Patent Application Laid-open Publication No. 2002-330035 (hereinafter referred to as “Patent Literature 1”)). In the case of the BTL connection, one speaker is bridge-connected to two class-D amplifiers of two channels, signals of opposite phases are input to the two class-D amplifiers, and the one speaker is driven differentially by the signals of opposite phases output from the two class-D amplifiers (see, for example, FIG. 4 of Patent Literature 1).
Further, among various types of conventionally-known class-D amplifiers is one where a common mode choke coil (CMC coil) is provided at a stage following a low-pass filter (see, for example, Japanese Patent Application Laid-open Publication No. 2003-124029 (hereinafter referred to as “Patent Literature 2”) and Japanese Patent Application Laid-open Publication No. 2003-46345 (hereinafter referred to as “Patent Literature 3”)). With the common mode choke coil provided at the stage following the low-pass filter, it is possible to remove noise components (common mode noise), such as high-frequency noise, that are transmitted in a common mode without having been removed by the low-pass filter. Note that the noise components transmitted in the common mode (common mode noise) are noise currents flowing in phase with each other through a hot-side output line and ground-side output line of the class-D amplifier. The common mode noise is included, for example, in switching noise produced by switching operation of a PWM modulation stage and an output stage. Therefore, in the class-D amplifier, measures against the common mode noise are important.
FIG. 5 is a diagram showing an example arrangement where a CMC coil is provided in a single-end connecting class-D amplifier. The CMC coil 50 includes two windings 52 and 53 wound on a single core 51 in the same phase direction, and one of the windings 52 is connected to the hot-side output of the class-D amplifier while the other winding 53 is connected to the ground-side output of the class-D amplifier in the same phase direction as the one winding 52. Thus, common mode currents (noise components), transmitted in the common mode, flow over the hot-side winding 52 and ground-side winding 53 in phase with each other, so that magnetic fluxes are produced in the two windings 52 and 53 in the same direction. Consequently, magnetic fluxes of the core 51 are added together to produce a great impedance. Thus, the CMC coil 50 prevents propagation of the common mode noise. For a current transmitted in a normal mode (i.e., output signal of the class-D amplifier), on the other hand, because a current flowing through the hot-side winding 52 to a speaker and a current flowing from the speaker to the ground-side winding 53 in an opposite direction to the former current counterbalance each other, magnetic fluxes produced in the hot-side winding 52 and the ground-side winding 53 cancel out each other. Thus, the CMC coil 50 permits passage therethrough of the output signal of the class-D amplifier without producing impedance to the current transmitted in the normal mode (i.e., output signal of the class-D amplifier). In this way, the core 51 of the CMC coil 50 is not saturated with magnetic fluxes with respect to the output signal of the class-D amplifier. In other words, since the great impedance to common mode noise due to saturation of magnetic fluxes is produced, only the common mode noise included in the output signal of the class-D amplifier can be effectively removed or reduced.
FIG. 6 is an example arrangement where a CMC coil is connected to class-D amplifiers designed for the BTL connection (hereinafter sometimes referred to as “BTL connecting class-D amplifiers”). In the BTL-connecting class-D amplifier, two windings 54 and 55 of the CMC coil are connected to respective outputs of the two class-D amplifiers. In the case of the BTL connection, respective output signals of the two class-D amplifiers are of opposite phases, and thus currents flowing through the windings 54 and 55 counterbalance each other so that magnetic fluxes produced in a core 56 cancel out each other. Thus, magnetic fluxes of the core 56 of the CMC coil 57 do not saturate with respect to the output signals of the class-D amplifiers. With respect to common noise included in the output signals of the class-D amplifiers, on the other hand, common mode noise current components of the same phase flow through the windings 54 and 55 to produce a great impedance, so that the common mode noise can be effectively removed.
FIG. 7A shows an arrangement where an audio amplifier (power amplifier) is constructed, using two single-end connecting class-D amplifiers each having a CMC coil as shown in FIG. 5, in such a manner that it is selectively switchable between the single-end connection mode for reproducing two-channel stereo signals and the BTL connection mode for reproducing a one-channel monaural signal, and where the thus-constructed audio amplifier is used in the single-end connection mode. In each of the CMC coils 62 and 63 provided in the two class-D amplifiers 60 and 61, currents flowing through the hot-side winding and ground-side winding counterbalance with each other so that magnetic fluxes do not saturate; thus, the CMC coils 62 and 63 can operate appropriately.
However, if the audio amplifier (power amplifier) shown in FIG. 7A is to be used in the BTL connection mode, a speaker is connected to hot-side output terminals 64 and 65 of the two class-D amplifiers 60 and 61 as shown in FIG. 7B. Thus, in this case, currents flow only through the hot-side windings of the CMC coils 62 and 63 provided in the two class-D amplifiers 60 and 61. Consequently, magnetic fluxes of the cores of the CMC coils 62 and 63 would saturate, so that the CMC coils 62 and 63 can not operate appropriately or effectively. Thus, with the prior art technique, the audio amplifier (power amplifier) provided with two single-end connecting class-D amplifiers having the respective CMC coils could not be used in the BTL connection mode.
Further, if two BTL-connecting class-D amplifiers having a CMC coil as shown in FIG. 6 are used in the single end connection mode where a speaker is driven independently per channel, the two class-D amplifiers output signals of different channels independently of each other, and thus, magnetic fluxes of the core 56 may saturate, for example, when electric currents of the same phase flow through the two windings 54 and 55 of the CMC coil 57, which would prevent the CMC coil 57 from operating effectively. Thus, heretofore, such usage of the class-D amplifiers has been impossible.
Namely, with the prior art technique, it has been impossible to construct a power amplifier (audio amplifier) which includes two class-D amplifiers and a CMC coil and of which a user can select as desired any one of two connection schemes or modes, i.e. single-end connection mode and BTL connection mode (i.e., which is switchable, through operation by the user, between the single-end connection mode and the BTL connection mode).